High solids hBN slurry, hBN paste, spherical hBN powder, and methods of making and using them

Abstract

The present invention relates to a method for making a hexagonal boron nitride slurry and the resulting slurry. The method involves mixing from about 0.5 wt. % to about 5 wt. % surfactant with about 30 wt. % to about 50 wt. % hexagonal boron nitride powder in a medium under conditions effective to produce a hexagonal boron nitride slurry. The present invention also relates to a method for making a spherical boron nitride powder and a method for making a hexagonal boron nitride paste using a hexagonal boron nitride slurry. Another aspect of the present invention relates to a hexagonal boron nitride paste including from about 60 wt. % to about 80 wt. % solid hexagonal boron nitride. Yet another aspect of the present invention relates to a spherical boron nitride powder, a polymer blend including a polymer and the spherical hexagonal boron nitride powder, and a system including such a polymer blend.

Description

and inlet and outlet temperatures had to be increased. Along with these changes, the flow rate of the slurry was slowed down and the revolutions per minute (rpm) of the atomizer increased. In addition, 4 wt. % glycerol was added into the slurry before spray drying if it was going to be used for dry pressing applications. During spray drying, the slurry was constantly mixed.

The inlet temperature was set to 235° C. which gave an outlet temperature of 85° C. The flow rate of the slurry was 60 ml/minute and the atomizer (Pentronix, Detroit, Mich.) was set at 12,500 rpm. These settings generally produced spherical BN powder in the size range of −150 μm/+30 μm. The lower end of the scale was quite variable depending on the dust collector damper setting. The powder collected had a moisture content of approximately 0.25-0.5%.

The slurry example outlined above required about 70 minutes to put through the spray dryer under these conditions. The powder yield was about 80% after screening out coarse particles, accounting for wall material, and material collected in the cyclone.

All of the conditions above are only valid for the spray dryer used in the present Example. Minor changes would be needed for work in any other system, which is expected. Larger dryers would allow more flexibility in particle size distribution and higher production rates.

The effect of wt. % boron nitride solids slurry loading on spray dried properties was then tested, as shown in Table 2.

TABLE 2
Effect of wt. % BN solids slurry loading on spray dried properties.
	Solids	LPD	Tap Density	Flow
Powder	(wt. %)	(g/cc)	(g/cc)	(sec)	Sizing (mm)
A	25	0.462	0.55	55.7	−150/+75
B	25	0.492	0.586	57.4	−75
C	25	n/a	0.541		−45
D	50	0.533	0.62	54	−75
E	50	0.574	0.652	43.2	−150
XP	n/a	0.44	0.562	75.3	−105/+74

Powders B and D, which were screened to the same size, showed that as solids loading increased, the density of the resulting spray dried powder increased.

Example 4

Production of BN Clay-Like Paste by the Slip Cast Method

Slurry from Example 1 was poured into a plaster slip cast mold. Pressure was applied and the set-up left to cast on the order of 12 hours. Because the molds were “blinded” so quickly, casting stopped and no more moisture was removed from the slip. The resultant material was a thick pasty material. The solids content was 76%.

Example 5

Production of BN Clay-Like Paste by the Vacuum Filtration Method

Slurry from Example 1 was poured into a Buchner Funnel with filter paper. A vacuum was pulled on the slurry from below. The water from the system flowed into a graduated flask. When the desired amount of water was removed from the slurry, the vacuum was removed. The BN paste sample, which had a solids content of 74%, was collected and sealed in an airtight bag for later use.

Although preferred embodiments have been depicted and described herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.

Claims

1. A method for making spherical boron nitride powder comprising:

providing a hexagonal boron nitride slurry;

spray drying the slurry under conditions effective to produce spherical boron nitride powder comprising spherical agglomerates of boron nitride platelets; and

sintering the spherical boron nitride powder.

2. The method according to claim 1, wherein the hexagonal boron nitride slurry comprises from about 30 wt. % to about 50 wt. % hexagonal boron nitride powder.

3. The method according to claim 1, wherein the spherical boron nitride powder has a tap density of about 0.4 g/cc to about 0.7 g/cc.

4. The method according to claim 1, wherein the sintering is carried out at a temperature of from about 1800° C. to about 2400° C.

5. The method according to claim 1, wherein the spherical agglomerates of boron nitride platelets have an average agglomerate diameter of from about 10 microns to about 500 microns.

6. The method according to claim 5, wherein the majority of boron nitride agglomerates have an average diameter of from about 30 microns to about 150 microns.

7. The method according to claim 1 further comprising;

classifying the spherical boron nitride powder under conditions effective to obtain a desired agglomerate size distribution.

8. The method according to claim 7, wherein the classifying is selected from the group consisting of screening, air classifying, and elutriation.

9. A spherical boron nitride powder comprising spherical agglomerates of boron nitride platelets.

10. The spherical boron nitride powder according to claim 9, wherein the spherical boron nitride powder has a tap density of about 0.4 g/cc to about 0.7 g/cc.

11. The spherical boron nitride powder according to claim 9, wherein the spherical agglomerates of boron nitride platelets have an average agglomerate diameter of from about 10 microns to about 500 microns.

12. The spherical boron nitride powder according to claim 11, wherein the majority of boron nitride agglomerates have an average diameter of from about 30 microns to about 150 microns.

13. A method for making a hexagonal boron nitride paste comprising:

providing a hexagonal boron nitride slurry and

treating the slurry under conditions effective to produce a hexagonal boron nitride paste comprising from about 60 wt. % to about 80 wt. % solid hexagonal boron nitride.

14. The method according to claim 13, wherein the hexagonal boron nitride slurry comprises from about 30 wt. % to about 50 wt. % hexagonal boron nitride solids loading.

15. The method according to claim 13, wherein said treating comprises placing the slurry in a plaster mold.

16. The method according to claim 13, wherein said treating comprises vacuum filtration.

17. A hexagonal boron nitride paste comprising from about 60 wt. % to about 80 wt. % solid hexagonal boron nitride in a medium.

18. The hexagonal boron nitride paste according to claim 17, wherein the medium is an aqueous medium.

19. The hexagonal boron nitride paste according to claim 18, wherein the medium is a non-aqueous medium selected from the group consisting of isopropyl alcohol, methanol, and ethanol.

20. A polymer blend comprising:

a polymer, and

a powder phase comprising spherical agglomerates of hexagonal boron nitride platelets, wherein the powder phase is distributed homogeneously within the polymer.

21. The polymer blend according to claim 20, wherein the powder phase has a tap density of about 0.4 g/cc to about 0.7 g/cc.

22. The polymer blend according to claim 20, wherein the polymer is selected from the group consisting of melt-processable polymers, polyesters, phenolics, silicone polymers, acrylics, waxes, thermoplastic polymers, low molecular weight fluids, and epoxy molding compounds.

23. The polymer blend according to claim 20, wherein the polymer blend comprises from about 30 wt. % to about 80 wt. % spherical boron nitride powder.

24. The polymer blend according to claim 20, wherein the polymer blend has a thermal conductivity of from about 1 W/mK to about 15 W/mK.

25. The polymer blend according to claim 20, wherein the spherical agglomerates of hexagonal boron nitride platelets have an average agglomerate diameter of from about 10 microns to about 500 microns.

26. The polymer blend according to claim 25, wherein the majority of spherical agglomerates have an average diameter of from about 30 microns to about 150 microns.

27. A system comprising:

a heat source;

a heat sink; and

a thermally conductive material connecting the heat source to the heat sink, wherein the thermally conductive material comprises a powder phase comprising spherical agglomerates of hexagonal boron nitride platelets.

28. The system according to claim 27, wherein the powder phase has a tap density of about 0.4 g/cc to about 0.7 g/cc.

29. The system according to claim 27, wherein the heat source is an integrated circuit chip, power module or transformer.

30. The system according to claim 27, wherein the heat sink is finned aluminum, copper, berilium or diamond.

31. The system according to claim 27, wherein the thermally conductive material comprises from about 30 wt. % to about 80 wt. % spherical boron nitride powder.

32. The system according to claim 27, wherein the thermally conductive material has a thermal conductivity of from about 1 W/mK to about 15 W/mK.

33. The system according to claim 27, wherein the spherical agglomerates of hexagonal boron nitride platelets have an average agglomerate diameter of from about 10 microns to about 500 microns.

34. The system according to claim 33, wherein the majority of spherical agglomerates have an average diameter of from about 30 microns to about 150 microns.

35. The system according to claim 27, wherein the thermally conductive material is a polymer.

36. The system according to claim 35, wherein the polymer is selected from the group consisting of melt-processable polymers, polyesters, phenolics, silicone polymers, acrylics, waxes, thermoplastic polymers, low molecular weight fluids, and epoxy molding compounds.

37. A spherical boron nitride powder comprising spherical agglomerates of hexagonal boron nitride platelets having an average agglomerate diameter of from 20 microns to 74 microns.

38. The spherical boron nitride powder according to claim 37, wherein the boron nitride agglomerates have an average diameter of from 38 microns to 74 microns.

39. The spherical boron nitride powder according to claim 38, wherein the boron nitride agglomerates have an average diameter of from 38 microns to 53 microns.

40. The spherical boron nitride powder according to claim 37, wherein the spherical boron nitride powder has a tap density of about 0.4 g/cc to about 0.7 g/cc.

41. A spherical boron nitride powder comprising spherical agglomerates of hexagonal boron nitride platelets having an average agglomerate diameter of from 10 microns to 38 microns.

42. The spherical boron nitride powder according to claim 41, wherein the boron nitride agglomerates have an average diameter of from 20 microns to 38 microns.

43. The spherical boron nitride powder according to claim 41, wherein the spherical boron nitride powder has a tap density of about 0.4 g/cc to about 0.7 g/cc.

44. A spherical boron nitride powder comprising spherical agglomerates of hexagonal boron nitride platelets, wherein the powder has an agglomerate size distribution, based on diameter of the agglomerates, of:

(i) 10 microns to 38 microns, or

(ii) 20 microns to 38 microns, or

(iii) 20 microns to 74 microns, or

(iv) 38 microns to 74 microns, or

(v) 38 microns to 53 microns.